Apparatus for and methods of sensing evanescent events in a fluid field

ABSTRACT

An evanescent filed based sensor uses a detector for sensing variations in properties of a fluid flowing in a boundary layer adjacent to the detector. The detector comprises an optical waveguide in the form of an optical fiber having a core layer covered by a cladding layer and having a substantially D-shaped cross section defining a planar surface with an optical grating pattern thereon. When a beam of laser light is directed through the detector as an input, variations in an output of the beam of laser light are indicative of changes in fluid pressure or density in the boundary layer or immediate region adjacent to the grating of the optical waveguide.

FIELD OF THE INVENTION

[0001] The present invention is directed to an apparatus for and methodsof sensing evanescent events in a fluid field. More particularly, thepresent invention is directed to such apparatus and methods using anevanescent field based fluid sensor which utilizes non-intrusive fiberoptic technology to sense hydrodynamic conditions.

BACKGROUND OF THE INVENTION

[0002] There is a need for sensors which detect hydrodynamic flowconditions, as well as fluid density conditions and variations, in amanner that reflect true conditions in that the sensor structure itselfdoes not interfere with fluid flow at the location being monitored. Forexample, in monitoring fluid flow conditions over an airfoil, it isadvantageous from both testing and fluid control purposes to know howthe fluid environment is interacting with the airfoil at a specific, butperhaps fleeting moment. This is because slight variations in fluiddynamic conditions can over even very short periods of time give rise tosituations of considerable interest. This is not only an issue inaerodynamics, but is also of great interest in medical applicationswhere the flow of blood through the circulatory system is monitored.This is because circulating blood is constantly changing in pressure,velocity and density as a myriad of physiological conditions react withthe blood stream.

[0003] The ability to detect fleeting changes in fluid flow conditionsis useful in many other situations, such as but not limited to, the flowof fluids in hypersensitive chemical processing plants and the flow ofgases through systems such as air conditioning ducts and gas scrubbingsystems. There are many situations in which maintenance of laminar fluidflow is important, such as air induction systems of internal combustionengines, wherein laminar flow of combustion air is important to maximizeefficiency in order to reduce pollutants and fuel consumption.

[0004] The need for non-intrusive, i.e., small, fluid sensors is alsoapparent in the marine industry in which vehicles are propelled throughtwo fluids simultaneously, i.e., air and water, which fluids areseparated by a very complex interface. Maximizing the efficiencies ofhydrodynamic surfaces on marine vessels requires knowledge of whatoccurs or is occurring at boundary layers directly adjacent to orperhaps even perhaps within skin structure defining the surfaces.

[0005] Further examples of the need to understand and thereby controlfluid flow over surfaces are exemplified by the need of next-generationlighter-than-air cargo and passenger air ships and by competition toimprove the effectiveness of sails on racing boats such as America's Cupyachts.

[0006] Currently, the complexities encountered when attempting tocomprehend boundary layer flow are perhaps best understood through threescalar partial differential equations that describe conservation ofmomentum for motion of a viscous, incompressible fluid. Thesecomplexities are frequently expressed mathematically in one complexexpression, which relates fluid density, fluid velocity, fluid pressure,body force, and fluid viscosity. This equation has few mathematicalsolutions. Thus, a sensor which effectively monitors boundary layerconditions would be of considerable assistance in coping with, andeffectively functioning within, an area of technology that hashistorically been extremely difficult to comprehend due to itscomplexity.

SUMMARY OF THE INVENTION

[0007] In view of the aforementioned considerations, a detector forsensing variations in properties of a fluid flowing in a boundary layeradjacent to the detector comprises an optical waveguide having a corecovered by a cladding. The optical waveguide has a planar surface withan optical grating pattern thereon. When a laser beam is directedthrough the detector, a probing beam is modulated by the grating in away which is indicative of changes in fluid properties in the boundarylayer adjacent to the grating.

[0008] In accordance with a more specific aspect of the invention, theoptical waveguide is an optical fiber with a D-shaped cross-section; theoptical fiber having the core disposed adjacent to the planar surfacewith the grating formed in the cladding adjacent to the core.

[0009] In accordance with a further aspect of the invention the gratinghas a first portion and a second portion, and in still a further aspectof the invention, the second portion is spaced from the first portion bya selected distance.

[0010] The invention may also be expressed as directed to a system forsensing variations in flow field intensity of a fluid flowing in aboundary layer adjacent to a body exposed to the fluid. The systemcomprises an optical fiber on or in the body, the optical fiber having acore covered by cladding and a D-shaped cross-section. The D-shapedcross-section defines a planar surface adjacent the core. The planarsurface has an optical grating thereon. A tunable laser produces a laserbeam which is directed through the optical fiber. Before passing throughthe optical fiber, the laser beam is directed through a beam splitterwhich produces a fiber probing beam and a reference beam. The fiberprobing beam passes through the optical fiber and interacts with theoptical grating while the reference beam is directed to a first sensorso as to produce a reference output indicative of the amplitude of thereference beam. A second sensor detects the fiber probe beam after ithas been modulated by the grating and produces a modulated outputindicative of the amplitude of the probe beam as modulated by thegrating. A comparator is connected to the first and second sensors forreceiving the reference output and the modulated output so as to producea differential signal indicative of the flow field intensity in theboundary layer adjacent to the body.

[0011] In further aspects of the invention, the tunable laser is anarrow linewidth, tunable laser which is passed through an opticalchopper disposed between the laser and the beam splitter. In stillfurther aspects of the invention, the first and second sensors arephotodiodes and the optical grating comprises at least first and secondgrating portions.

[0012] The invention is also directed to methods for sensing variationsin properties of a fluid flowing in a boundary layer adjacent to adetector, wherein the method comprises directing a beam of laser lightthrough an optical waveguide. The optical waveguide has a core layercovered by a cladding layer defining a planar surface with an opticalgrating pattern thereon. Variations in an output of the beam of laserlight are detected, which variations are indicative of changes in fluidpressure or on density in the boundary layer adjacent to the grating ofthe optical waveguide.

[0013] The method further comprises configuring the optical waveguide asan optical fiber with a D-shaped cross-section.

[0014] In a more specific aspect of the method, the optical fiber has anoptical grating with first and second portions having line spacingscorresponding to first and second Bragg angles, respectively. The laserbeam is forward coupled by the first portion and forward and reversedcoupled by the second portion to sense fluid conditions in the boundarylayer so as to modulate the laser beam output and to also provide areference beam.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Various features and attendant advantages of the presentinvention will be more fully appreciated as the same becomes betterunderstood when considered in conjunction with the accompanyingdrawings, in which like reference characters designate the same orsimilar parts throughout the several views, and wherein:

[0016]FIG. 1 is a perspective view showing a detector configured inaccordance with the principles of the present invention for sensingvariations in properties of a fluid;

[0017]FIG. 2 is a planar view of a corrugated optical grating;

[0018]FIG. 3 is a diagrammatic illustration of a system for sensingvariations in flow field intensity of a fluid utilizing the detector ofFIG. 1;

[0019]FIG. 4 is a perspective view of a second embodiment of a detectorconfigured in accordance with the principles of the present invention;

[0020]FIG. 5 is a schematic view of a system for sensing variations inflow field intensity or dynamic index variations of a fluid utilizingthe detector of FIG. 4, and

[0021]FIG. 6 is a schematic view similar to FIG. 5 but showing a thirdembodiment of a detector in accordance with the principles of thepresent invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0022] Referring now to FIG. 1, there is shown optical detectorarrangement 10, configured in accordance with the principles of thepresent invention, for sensing variations in intensity of a fluid flowfield 12 disposed adjacent to a body 14. In the illustrated embodiment,the optical detector arrangement 10 preferably utilizes an optical fiber16 comprised of a core 18 surrounded by cladding 20. While an opticalfiber 16 is preferred as a detector, other configurations could be used,for example, wafers which could be rectangular, round or have any shapewhich would perform according to the principles of the presentinvention.

[0023] As seen in FIG. 1, the optical fiber 16 has a D-shapedcross-sectional profile with a semi-cylindrical or elliptical surface 22and a planar surface 24. Planar surface 24 is disposed adjacent to thefluid flow field 12 and has an optical grating 26 in a portion 28 of thecladding 20 that overlies the core 18 in a region 30 of the core. Thegrating 26 has lines 32 spaced a selected distance apart which result inBragg angle reflections or resonant mode coupling for a selected Braggwavelength λ_(Bragg). In the example of FIG. 1, the grating 26 is acorrugation formed by ablation of the cladding 20, however the gratingmay also be formed optically by photo-induced index changes.

[0024] The D-shaped optical fiber 16 is preferably mounted in a V-shapedgroove 33 in a body 38 with a locally planar surface 40, the D-shapedoptical fiber 16 having only its planar surface 24 exposed to the flowfield 12 so that the planar surface 40 of the body 38 is coincident, ifnot parallel with the planar surface 24 of the optical fiber. As is setforth earlier in this application, the body 38 can be any body overwhich the fluid 12 flows, such as but not limited to an air foil, asurface of a ship, a submarine, a medical instrument, a sail, or anyother instrumentality. Since the fiber 16 is flush with the surface 40of the body 14, the resulting sensor is non-intrusive and can measureproperties of hydrodynamic flow, fluid density and phase change bydetecting minute variations in actual boundary layer conditions. Thus,through evanescent coupling, fleeting changes in intensity of the flowfield 12 are detectable.

[0025] The sensor of FIG. 1 is designed such that in the stationarycondition, i.e. no fluid flow, nearly 100% of the guided mode, i.e.passage of light through the core 18, is coupled out of the opticalfiber 16 in the vicinity of the grating 26. The responses of the sensingarrangement 10 to pressure changes under both subsonic and hypersonicflow conditions are related to induced periodic boundary conditionsimposed on the D-fiber structure by fabrication of the grating. Thesesame responses are governed by Bernoulli conditions within the boundarylayer adjacent to the optical fiber 16. Since the relationship betweenpressure and fluid density within the boundary layer affects theevanescent filed coupling between the guided and unguided modes of theoptical fiber 16 i.e. the core 18 and cladding 20, respectively, smalldeviations from a resulting output null are directly related to pressuredifferentials.

[0026] Referring now to FIG. 3, the detector arrangement 10 of FIG. 1,comprising the optical fiber 16 and the body 38, is utilized incombination with a system 42 for detecting variations in a fluid flowfield 12. In the system 42 of FIG. 3, a narrow linewidth, tunable laser46 having automated, wavelength scanning capabilities is employed todirect a laser beam 48 through an optical chopper 50 and beam splitter52. The laser 46 is a single frequency laser, tuned to the Braggresonance of the optical fiber 16 set by the Bragg wavelength λ_(Bragg)in order to generate a null during stationary state conditions in whichthere is no flow field 12 i.e. when the velocity of the fluid flow fieldis zero. Ideally, when in the null condition, there is no light exitingthe fiber because there is complete evanescent field coupling of theguided light in the core 18 with the unguided light in the cladding 20due to subsequent coupling outside of the fiber. The null conditionoccurs in the immediate region 30 of the periodic patterns formed by thegrating 26 before the light reaches the exit end of the optical fiber.

[0027] In accordance with the present invention, the beam splitter 52provides a 10 reference beam 58 which is sensed by a first photodiode60. The output 61 of the first photodiode 60 is transmitted to adifferential input, lock-in amplifier 64. The beam splitter 52 alsoprovides a probing beam 66 which passes through the optical fiber 16 andinteracts with the grating 26 in the cladding 20 while being guidedthrough the core 18. Thus a modulated probing beam 66′ is detected by asecond diode 67 which has an output 68 proportional to the amplitude ofthe modulated probing beam 66′. The output 68 of the second diode 67 istransmitted to the differential input, lock-in amplifier 64 where itsamplitude is compared to that of the output 61 from the diode 60 whichsenses the reference beam 58. The lock-in amplifier 64 has an outputsignal 69 which is transmitted to a monitoring circuit 70. Themonitoring circuit 70 may provide any number of functions which relateto the body 38, such as but not limited to controlling the body 38 orsome related element with respect to the field flow, displayingvariations in flow field intensity or storing detected conditions forlater review and use.

[0028] When the outputs 61 and 68 of the first and second photodiodes 60and 67 cannot be made to match, the output 69 to the monitoring circuit70 is not a null. Rather, the output 69 is a signal having an intensityproportional to the difference in amplitude between the output 68 of thesecond photodiode 67 which detects the modulated signal 66′ and theamplitude of the reference signal 61 from the first photodiode 60. Sincepressure or density are direct functions of changes in flow fieldintensity 12, the monitor 70 can utilize Bernoulli's law to determinethe speed of the flow field 12 over the body 38. The arrangement canalso be used to sense a change in state. For example, if the fluid flowfield changes from air and water vapor to ice on an airfoil surface 40,the boundary layer is no longer adjacent the airfoil surface. In itsstead is a substance (ice) of a markedly different index of refractionso that the detection system 10 generates an immediate output notifyingan aircraft pilot that ice has formed on an airfoil. Another example ofa change in state occurs in liquids where there can be an abrupt changein pressure due to formations of cavities within liquids adjacent asolid surface.

[0029] The material composition of the optical fiber 16 of FIGS. 1-3 ishigh grade fused silica over-cladding 20 with a fluorine-based silicacladding and a Ge and F-doped core region 18 beneath (roughly 10 μm for820 nm D-fiber) the flat portion of the fiber 16. Due to the generalnature of glass, the dimensions of D-fibers can be easily varied duringtheir manufacture. However, due to typical wavelengths requirementsassociated with Ge-doped optical waveguides, the optical fiber 16 isgenerally made in a variety of five commercial dimensions correspondingto five key fiber and/or laser source wavelengths. These wavelengths,technically known as cutoff wavelength are 550±60 nm (allowing thesingle mode operation of the He—Ne 633 nm gas and the 670 nmsemiconductor lasers), 700±60 nm (corresponding to semiconductor lasersat 820 nm), 890±70 nm (corresponding to semiconductor and fiber lasersat 890, 980, and 1060 nm), 1040±170 nm (corresponding to 1300 nmsemiconductor lasers and the first low loss transmission window oftelecommunication-grade Ge-doped optical fibers), and 1290±70 nm cutoff(corresponding to the 1500-1550 nm range semiconductor lasers and thelowest loss transmission window for Ge-doped optical waveguides). Thecutoff wavelength is the wavelength below which single mode operation isno longer possible. Therefore, the single mode operating bandscorresponding to the cutoff wavelengths given above are: 610-700,760-900, 960-1250, 1110-1400, and 1360-1680 nm, respectively. The fiberdiameters corresponding to these five cutoff wavelengths are 70, 80,125, 125, and 125 μm, respectively.

[0030] Exemplary of a non-intrusive configuration for the hydrodynamicdetector 10 using a D-fiber for 1550 nm operation is a D-fiber having across-sectional diameter of 125 μm (O.D.) and a flat width of 121 μm.The core 18 is located 16 nm from the planar surface 24 of the opticalfiber 16. Since optical fibers 16 of various dimensions are made fromsimilar fiber preforms with high dimensional tolerances, allmeasurements scale proportionately for the various fiber diameters.

[0031] Considering now the corrugation spacing of gratings, it isevident from the nature of electromagnetic mode coupling that thewavelength parameters are strongly dependent upon pattern depth, sincethe evanescent field associated with the single guided mode in questiondiminishes quite rapidly and requires proximity interaction with thesecorrugated patterns or gratings 26. This is so because in all practicalimplementations of this device it is desirable to couple (or null) theguided modes over as short a distance as possible. The following are twoexamples of parameter sets associated with coupling 100% of the lightout of the fiber core 18 at steady state. Example 1: Nominal Mixinghalf-Length 675.858 μm Corrugation depth 0.2 μm Spacing 19.5791 μmNominal sensed index 1.37 Nominal Laser Wavelength 1.52 μm Example 2:Nominal Mixing Half-Length 292.625 μm Corrugation depth 0.5 μm Spacing19.5791 μm Nominal sensed index 1.37 Nominal Laser Wavelength 1.52 μm

[0032] Since the resonance is very sharp, it is important for apractical device to use a tunable laser source such as a tunablesemiconductor unit operating in the 1500 to 1550 nm wavelength range.This principle translates to any tunable or nontunable systems capableof allowing the joint conditions of being on resonance and initiating aguided fiber mode that is resonant with an unguided mode through theinteraction of its evanescent field with these corrugations (in the caseof the etched or ablated patterns in question).

[0033] Referring now to FIGS. 4 and 5 there is shown, optical fiber 100configured in accordance with the principles of the present invention.The optical fiber 100 is D-shaped in a manner similar to the opticalfiber 16 of FIGS. 1-3 and is preferably mounted on (or rather in) a body38 with only its planar surface 24 exposed. Instead of having singleoptical grating 26 as in FIGS. 1-3, the optical fiber 100 of FIGS. 4 and5 has a pair of optical grating portions 102 and 104. The gratingportions 102 and 104 are shown separate by a gap 106. As with theoptical fiber 16, the optical fiber 100 has core 18 and cladding 20.Preferably, the periodic variations forming the gratings 102 and 104 areproduced by photo-induced index of refraction modulations and thus areless fragile and more ameanable to multi-pattern interactions and sensordesign modifications.

[0034] The first grating portion 102 has spacing defined by thewavelength λ_(Bragg (Core-->Clad)) while the second grating portion 104has spacing defined by the wavelength λ_(Bragg (Clad<--Clad)). This dualpass arrangement effectively doubles the interaction length in the gap106 and thus heightens the sensitivity of the detector.

[0035] The optical fiber of 100 of FIG. 4 is used in combination with asystem 110 of FIG. 5 for detecting variations in the fluid flow field 12by utilizing back reflections within the optical fiber 100. As is seenin FIG. 5, a diode laser 112 is disposed at an angle α with respect tothe input face 114 of the optical fiber 100 in order to eliminatespurious signals 115′ that could originate from the fiber input end. Inthe embodiment of FIG. 5, the input base 114 is disposed obliquely withrespect to the axis of optical fiber 100. This allows the gratinginduced back-reflection phenomena of laser beam 115 illustrated by theblack arrows 116, with portions of forward coupling, illustrated bywhite arrows 117, to be void all modulating effects except the actualsignals of interest. The following relationship results for Braggpattern spacing in the case of back reflection in a guiding medium:$\Lambda_{B} = {\left. \frac{\pi}{\beta}\Rightarrow\Lambda_{B_{c}} \right. = {{\frac{\pi}{\beta_{c}}\quad {and}\quad \Lambda_{B_{cl}}} = \frac{\pi}{\beta_{cl}}}}$

[0036] where c and cl are the fiber core 18 and cladding 20 indices,respectively. Similarly, for core-clad forward coupling, the followingrelationship holds:${\Lambda_{B_{c\rightarrow{cl}}} = {{\frac{2\pi}{\beta_{c} - \beta_{cl}}\quad {or}\quad \frac{1}{\Lambda_{B_{c\rightarrow{cl}}}}} = {{\frac{1}{2\Lambda_{B_{c}}} - \frac{1}{2\Lambda_{B_{cl}}}} = {\frac{\beta_{c}}{2\pi} - \frac{\beta_{cl}}{2\pi}}}}},{{\therefore\frac{2}{\Lambda_{B_{c - {cl}}}}} = {\frac{1}{\Lambda_{B_{c}}} - {\frac{1}{\Lambda_{B_{cl}}}.}}}$

[0037] Note that Λ is approximately equal n k_(vac)/π so that the lastexpression can be rewritten as

[0038] Λ_(B) _(c→cl) ≧2π/(n_(c)−2n_(Sensed))

[0039] As is seen in FIG. 5, sensing signal system 110 functionssomewhat similar to the system of FIG. 3 with a first photodiode 118providing the dominant modulated signal output proportional to theamplitude of the grating induced back reflected probe beam 116 and asecond photodiode 119 providing a conjugate output proportional to theamplitude of the forward coupled portion of the modulated probe beam120′ which has been and passed through the optical fiber 100. In FIG. 5,a forward coupled beam 120 is transmitted through the cladding layer 20so as to emit fluid dependent radiation 130 which is affected byboundary conditions 12 adjacent the planar surface 24 of the opticalfiber 100, while the back reflected beam 120″ in the cladding 20 emitssimilar fluid dependent radiation as it retraces the path of beam 120and is not transmitted out of the end 132 of optical fiber 100. Due tocoupling of the beam 120′ transmitted though the cladding 20 and thesecond grating 104, the beam 120′ is minimally forward coupled out ofthe optical fiber 100 and is detected and measured by the photodiode119, its amplitude having been diminished substantially by fluidconditions in the boundary layer 12 adjacent to the cladding 20.

[0040] Similar in setup but somewhat different from the arrangement ofFIG. 3, the first photodiode 118 and the second photodiode 119 haveoutputs 135 and 137. These could be respectively connected to the inputand/or normalization channels of a lock-in amplifier 138. This wouldallow the detection of amplitude modulations of the outputs 135 and 137to produce a signal 139 that is proportional to the dynamic variationsin fluid density or fluid pressure in the boundary layer or the fluidregion adjacent to the optical fiber 100. The signal 139 is thentransmitted to a monitoring circuit 140 which functions similar to themonitoring circuit 70 of FIG. 3.

[0041] The following derivations pertain to coupled mode gratingformulations for the optical waveguide sensor 100 of FIGS. 4 and 5.

[0042] Starting with the wave equation for a perturbed dielectric mediumwe have,${\nabla^{2}E} = {{{\frac{1}{c^{2}}\left\lbrack {ɛ + {\delta ɛ}} \right\rbrack}\frac{\partial^{2}E}{\partial t^{2}}\quad {and}\quad E} = {{E\left( {r,z,t} \right)} = {{Re}\left\{ {\sum\limits_{m}\quad {\left\lbrack {{{A_{m}^{(t)}(z)}^{{\beta}_{m}z}} + {{A_{m}^{( - )}(z)}^{{- {\beta}_{m}}z}}} \right\rbrack {\xi_{m}(r)}^{{- {\omega}}\quad t}}} \right\}}}}$

[0043] where r is the transverse coordinate, m the mode number, A⁽⁺⁾ andA⁽⁻⁾ are the respective forward and reverse moving field amplitudes,ξ_(m) are the unperturbed mode eigenfunctions, and

[0044] δε(

, z)=δn²(r)cos(2πz/Λ) is the “pattern written in the fiber”.

[0045] For reflections of the m^(th) mode,$\frac{A_{m}^{( \pm )}}{z} = {{\pm i}\quad \kappa_{m}^{{- 2}{i{({\beta_{m} - {\pi/\Lambda}})}}z}A_{m}^{( \mp )}}$

[0046] is our “coupled differential equation” with the couplingparameter given by$\kappa_{m} = {\left( \frac{\omega}{c} \right)^{2}{\int{\delta \quad {n^{2}(r)}{{\xi_{m}\left( \overset{\rightharpoonup}{r} \right)}}^{2}{{^{2}r}/2}\beta {\int{{{\xi_{m}\left( \overset{\rightharpoonup}{r} \right)}}^{2}{{^{2}r}.}}}}}}$

[0047] Recalling that the condition for resonance reflection is given byΛ=π/μ_(m) and

[0048] A_(m) ⁽⁻⁾(z=L)=0 where L is the pattern length.

[0049] Thus the forward and reverse mode amplitudes are given by thefollowing expressions:

[0050] A_(m) ⁽⁺⁾(z)=A_(m) ⁽⁺⁾(0)[cos h(κ_(m)z)+tan h(κ_(m)L)sinh(κ_(m)z)] and

[0051] A^(m) ⁽⁻⁾(z)=iA_(m) ⁽⁺⁾(0)[sin h(κ_(m)z)+tan h(κ_(m)L) cosh(κ_(m)z)].

[0052] The forward propagating modes obey the following equations:$\frac{{A_{m}^{( + )}(z)}}{z} \approx {{i\left\lbrack {{\sum\limits_{s}\quad {\kappa_{m\quad s}^{{{({\beta_{s} - \beta_{m} + {2{\pi/\Lambda}}})}}z}{A_{s}^{( + )}(z)}}} + {\int{{K_{m}(q)}^{{{({q_{s} - \beta_{m} + {2{\pi/\Lambda}}})}}z}{A_{q}(z)}{q}}}} \right\rbrack}\quad {where}}$$\kappa_{m\quad s} = {\left( \frac{\omega}{c} \right)^{2}{\int{\delta \quad {n^{2}(r)}{\xi_{m}\left( \overset{\rightharpoonup}{r} \right)}{\xi_{s}\left( \overset{\rightharpoonup}{r} \right)}{{^{2}r}/\left( {4\beta_{m}\beta_{s}{\int{{{\xi_{m}\left( \overset{\rightharpoonup}{r} \right)}}^{2}{^{2}r}{\int{{{\xi_{s}\left( \overset{\rightharpoonup}{r} \right)}}^{2}{^{2}r}}}}}} \right)^{1/2}}}}}$

[0053] where the first term on the right hand side of the equationrepresents discrete modes and the second term denotes continuum orradiation modes. There are similar expressions for mode mixing betweentwo continuum modes. Finally, for resonant mode mixing between theguided mode with propagation constant β_(o) and an arbitrary radiationmode with propagation constant β_(r) where β_(o)−β_(r)=2π/Λ thefollowing expressions can be written assuming no radiation losses andinitial condition A_(r) ⁽⁺⁾(0)=0:${A_{o}^{( + )}(z)} = {{A_{o}^{( + )}(0)}{\cos \left\lbrack {\frac{1}{2}\left( \frac{\omega}{c} \right)^{2}\frac{\kappa_{or}z}{\sqrt{\beta_{o}\beta_{r}}}} \right\rbrack}}$${A_{r}^{( + )}(z)} = {i\sqrt{\frac{\beta_{o}}{\beta_{r}}}{A_{o}^{( + )}(0)}{{\sin \left\lbrack {\frac{1}{2}\left( \frac{\omega}{c} \right)^{2}\frac{\kappa_{or}z}{\sqrt{\beta_{o}\beta_{r}}}} \right\rbrack}.}}$

[0054] The forward and reverse moving field amplitudes A_(o) and A_(r)correspond to the amplitudes of the modulated probe beam 120 and backreflected beam 116 of FIG. 5 which produce voltage outputs 135 and 137which are compared in the differential input, lock-in amplifier 138.

[0055] In FIG. 6 an optical grating 150 is slanted with respect to theplanar surface 152 of the optical fiber 154, for example, preferably atan angle of 45°. In this embodiment an anti-reflective coating ordielectric layer 156 made of a material such as, for example, magnesiumfluoride (MgF₂), is disposed over the cladding 158. The dielectric layer156 has a thickness of one half the wavelength of the beam to increasereflection back into the optical fiber 154. A beam dump 160 to minimizebeam reflection is disposed at the end 162 of the optical fiber 154.This arrangement is suitable for situations in which the optical fiberindex n substantially equals 1.414.

[0056] This invention describes fiber optic sensing devices, based uponevanescent field coupling in a D-shaped fiber with periodic patterns. Inparticular, these patterns are either created by physical ablation(resulting in the removal of material) or by photo-induced indexchanges. In either case, the objective is the coupling of light out ofthe guided region or core 18 of a fiber 16 or 100 and into the unguidedregions or cladding 20 in order to null the light throughput resultingfrom normal guidance. This leads to a situation in which externalchanges in the outer vicinity of the fiber strongly influence the nullstate and give rise to straightforward extraction of informationconcerning dynamic states in regions immediately external to the opticalfiber 10 or 100 such as the boundary layer 12. Since the change in thenull condition is directly related to locally external environment(i.e., only in locations where the periodic patterns exist), thisinformation can be transmitted to a remote observer at either end of theoptical fiber 10 or 100.

[0057] From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

1. A detector for sensing variations in properties of a fluid flowing ina boundary layer adjacent to the detector, the detector comprising anoptical waveguide having a core layer covered by a cladding layerdefining a planar surface with an optical grating pattern thereon,whereby when a beam of laser light is directed through the detector asan input, variations in an output of the beam of laser light areindicative changes in fluid pressure or density in the boundary layeradjacent to the grating of the optical waveguide.
 2. A detectoraccording to claim 1 wherein the optical waveguide is an optical fiberwith a D-shaped cross section defining a planar surface and wherein thecore is adjacent to the planar surface and the grating is formed in thecladding.
 3. A detector according to claim 2 wherein the grating has afirst portion and a second portion, the second portion being spaced fromthe first portion by a selected distance.
 4. A detector according toclaim 2 wherein the optical grating pattern is slanted at an angle withrespect to the planar surface of the fiber.
 5. A detector according toclaim 4 wherein the angle is 45°.
 6. A system for sensing variations inflow field intensity of a fluid flowing in a boundary layer adjacent toa body exposed to the fluid, the system comprising: an optical fiber inor on the body, the optical fiber having at least an input, face and anoutput face and a core covered by cladding; the optical fiber havingD-shaped cross-section defining a planar surface adjacent to the core,the planar surface having an optical grating thereon; a tunable laserfor producing an initial laser beam; a beam splitter disposed betweenthe turnable laser and an input end of the optical fiber for providing aprobing beam and a reference beam, wherein the probing beam passesthrough the optical fiber for interaction with the optical grating; atleast a first detector for receiving the reference beam and producing anoutput indicative of the amplitude of the the reference beam; a secondsensor receiving the probe beam as modulated by variations in flow fieldintensity for producing a modulated output indicative of the amplitudeof the probe beam as modulated by the grating; and a comparatorconnected to the first and second sensors for receiving the referenceoutput and the modulated output and for producing a differential signalindicative of flow field intensity in the boundary layer adjacent to thebody.
 7. The system of claim 6 wherein the tunable laser is a narrowlinewidth tunable laser and wherein an optical chopper is disposedbetween the laser and the beam splitter.
 8. The system of claim 6wherein the first and second sensors are photodiodes.
 9. The system ofclaim 6 wherein the optical grating is in the core of the optical fiberand comprises at least a first portion and second portion.
 10. Thesystem of claim 6 wherein the initial laser beam is oriented at an anglewith respect to the input end face of the optical fiber and the gratingis slanted at an angle with respect to the planar surface of the opticalfiber.
 11. The system of claim 6 wherein the optical grating has firstand second portions with the first portion having a line spacingcorresponding to a first Bragg angle for forward coupling the initiallaser beam through the cladding and thus into the second portion of theoptical grating; the second portion having a line spacing correspondingto a second Bragg angle coupling for reverse coupling of the laser beamas a reverse laser beam back into the cladding adjacent to the boundarylayer and back to the first grating, which first grating throughreciprocity couples the reverse laser beam back into the core and out ofthe entrance face of the fiber for signal detection by the first sensor;the line spacing of the second portion also corresponding to the secondBragg angle configured to minimally forward couple the laser beam backto the core for transmission out of the optical fiber as a probing beamsensed by the second sensor.
 12. The system of claim 11 wherein theinput end face is disposed at an angle with respect to the longitudinalaxis of the optical fiber in order to minimize unwanted front endreflections not necessarily associated with modulated signals ofinterest.
 13. A method for sensing variations in properties of a fluidflowing in a boundary layer adjacent to a detector, the methodcomprising: directing a beam of laser light through an optical waveguidehaving a core layer covered by a cladding layer and defining a planarsurface with an optical grating pattern thereon, and detectingvariations in an output of the beam of laser light indicative changes influid pressure or density in the boundary layer adjacent to the gratingof the optical waveguide.
 14. A method according to claim 13 wherein theoptical waveguide is an optical fiber with a D-shaped cross section andwherein the core is adjacent to the planar surface and the gratingpattern is formed in the cladding.
 15. A method according to claim 14wherein the grating pattern has a first portion and a second portion,the second portion being spaced from the first portion by a selecteddistance.
 16. A method of using an optical fiber for sensing variationsin flow field intensity of a fluid flowing in a boundary layer adjacentto a body exposed to the fluid, the method comprising: producing aninitial laser beam with a tunable laser; splitting the laser beam withbeam splitter disposed between the turnable laser and an input end of anoptical fiber for providing a probing beam and a reference beam;directing the probing beam through an optical fiber in or on the body,the optical fiber having at least an input face, an output face, a corecovered by cladding and a D-shaped cross-section defining a planarsurface adjacent to the core, the planar surface having an opticalgrating thereon; detecting the reference beam and producing a referenceoutput indicative of the amplitude thereof; detecting the probe beam asmodulated by the conditions in the boundary layer for producing amodulated output indicative of the amplitude of the probe beam asmodulated by the grating; and comparing the reference output and themodulated output to produce a differential signal indicative of flowfield intensity in the boundary layer adjacent to the body.
 17. Themethod of claim 16 wherein the laser beam is a narrow linewidth beamfrom a tunable laser and further including chopping the laser beambefore splitting the laser beam.
 18. The method of claim 16 wherein thesensors are photodiodes.
 19. The method of claim 16 wherein the opticalgrating is in the core of the optical fiber and comprises at least afirst portion and a second portion.
 20. The method of claim 16 whereinthe initial laser beam is oriented at an angle with respect to the inputend face of the optical fiber.
 21. The method of claim 16 wherein theoptical grating has first and second portions with line spacingscorresponding to first and second Bragg angles, respectively; the methodcomprising: forward coupling the initial laser beam through the firstportion of the optical grating into the second portion of the opticalgrating; using the second portion of the grating to reverse couple thelaser beam into the cladding adjacent to the boundary layer and sendinga reverse laser beam back to the first portion of the grating; throughreciprocity, coupling the reverse laser beam back into the core and outof the inlet face to provide the reference beam for signal detection bythe first sensor, while minimally forward coupling a forward laser beamback to the core for transmission out of the optical fiber to provide amodulated probe beam for signal detection by the second sensor.
 22. Themethod of claim 21 comprising minimizing unwanted front end reflectionsnot associated with the modulated signals of interest by disposing theinput end face of the optical fiber at an angle with respect to thelongitudinal axis of the optical fiber.